1. Field of the Invention
The present invention relates to an improved method to produce a useful electrical charge storage device. The device, a capacitor, has improved space requirements (small size and true bipolar construction), low current leakage, low equivalent series resistance, high voltage per cell, high total voltage, high charge capacity and a long term useful lifetime.
2. Description of the Related Art
There has been significant research over the years regarding the obtaining of a useful reliable electrical storage device, such as a capacitor or a battery. Large energy storage capabilities are common for batteries; however, batteries also display low power densities. In contrast, electrolytic capacitors possess very high power densities and a limited energy density. On the other hand, carbon based double-layer capacitors have a large energy density; but, due to their high equivalent series resistance (ESR), carbon electrodes have low power capabilities. It would therefore be highly desirable to have an electrical storage device that had both large energy and power densities.
A recent review by B. E. Conway in J. Electrochem. Soc., Vol. 138 (#6), p. 1539 (June 1991) discusses the transition from "supercapacitor" to "battery" in electrochemical energy storage. He also identifies performance characteristics of various capacitor devices.
D. Craig, Canadian Patent No. 1,196,683, issued in November 1985, discusses the usefulness of electric storage devices based on ceramic-oxide coated electrodes and pseudo-capacitance. However, attempts to utilize this disclosure have resulted in capacitors which have inconsistent electrical properties and which are often unreliable. These devices cannot be charged to 1.0 V/cell and have large, unsatisfactory leakage currents. Furthermore, these devices have a very low cycle life. In addition, the disclosed packaging is inefficient.
M. Matroka and R. Hackbart, U.S. Pat. No. 5,121,288, issued Jun. 9, 1992, discusses a capacitive power supply based on the Craig patent which is not enabling for the present invention. A capacitor configuration as a power supply for a radiotelephone is taught; however, no enabling disclosure for the capacitor is taught.
J. Kalenowsky, U.S. Pat. No. 5,063,340, issued Nov. 5, 1991, discusses a capacitive power supply having a charge equalization circuit. This circuit allows a multicell capacitor to be charged without overcharging the individual cells. The present invention does not require a charge equalization circuit to fully charge a multicell stack configuration without overcharging an intermediate cell.
H. Lee, et al. in IEEE Transactions on Magnetics, vol 25 (#1), p.324 (January 1989) and G. Bullard, et al., in IEEE Transactions on Magnetics, vol.25 (#1) p. 102 (January 1989) discuss the pulse power characteristics of high-energy ceramic-oxide based double-layer capacitors. In this reference various performance characteristics are discussed, with no enabling discussion of the construction methodology. The present invention provides a more reliable device with more efficient packaging.
Carbon electrode based double-layer capacitors have been extensively developed based on the original work of Rightmire U.S. Pat. No. 3,288,641. A. Yoshida et al., in IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. CHMT-10, #1,P-100-103 (March 1987) discusses an electric double-layer capacitor composed of activated carbon fiber electrodes and a nonaqueous electrolyte. In addition, the packaging of this double-layer capacitor is revealed. These devices are on the order of 0.4-1 cc in volume with an energy storage capability of around 1-10 J/cc.
T. Suzuki, et al., in NEC Research and Development, No. 82, pp. 118-123, July 1986, discloses improved self-discharge characteristics of the carbon electric double-layer capacitor with the use of porous separator materials on the order of 0.004 inches thick. An inherent problem of carbon based electrodes is the low conductivity of the material resulting in a low current density being delivered from these devices. A second difficulty is that the construction of multicell stacks is not done in a true bipolar electrode configuration. This results in inefficient packaging and lower energy and power density values.
Additional references of interest include, for example:
The state of solid state micro power sources is reviewed by S. Sekido in Solid State Ionics, Vol. 9, 10, pp. 777-782 (1983). M. Pham-Thi et al. in the Journal of Materials Science Letters, Vol. 5, p. 415 (1986) discusses the percolation threshold and interface optimization in carbon based solid electrolyte double-layer capacitors.
Various disclosures discuss the fabrication of oxide coated electrodes and the application of these electrodes in the chlor-alkali industry for the electrochemical generation of chlorine. See for example: V. Hock, et al. U.S. Pat. No. 5,055,169 issued Oct. 8, 1991; H. Beer U.S. Pat. No. 4,052,271 issued Oct. 4, 1977; and A. Martinsons, et al. U.S. Pat. No. 3,562,008 issued Feb. 9, 1971. These electrodes, however, in general do not have the high surface areas required for an efficient double-layer capacitor electrode.
Additional references of general interest include U.S. Pat. Nos. 3,718,551; 4,052,271; and 5,055,169.
None of these references individually or collectively teach or suggest the present invention.
All of these applications, patents, articles, references, standards, etc. cited in this application are incorporated herein by reference in their entirety.
It would be very useful to have an improved method to produce a reliable long-term electrical storage device and the device thereof. The present invention provides such an improved method and improved device with energy densities of at least 20-90 J/cc.